The instant invention is in the field of sensors and more specifically the instant invention is in the field of wireless sensors employing harmonic electromagnetic transponders.
A harmonic electromagnetic transponder receives electromagnetic radiation at a fundamental frequency and emits electromagnetic radiation at a harmonic of the fundamental frequency. The harmonic of the fundamental frequency is usually the first harmonic (or twice the fundamental frequency). A passive harmonic electromagnetic transponder requires no power source (other than the power of the incoming electromagnetic radiation) for its operation. An active harmonic electromagnetic transponder requires a power source for its operation. As discussed by Maas, The RF and Microwave Circuit Design Cookbook, 1998, Chapter 2 , a common passive harmonic electromagnetic transponder comprises a receiving antenna tuned to resonate at the frequency of the incoming electromagnetic radiation, an emitting antenna tuned to resonate at twice the frequency of the incoming electromagnetic radiation, the receiving antenna being electrically connected to the emitting antenna by a Schottky diode. The electromagnetic radiation is usually in the radio wave or “radar” portion of the electromagnetic spectrum.
U.S. Pat. No. 3,781,879 described a harmonic radar detecting and ranging system for automotive vehicles wherein the receiving and emitting antennas are arranged for orthogonal polarization of the received and emitted electromagnetic radiation. U.S. Pat. No. 4,001,822 described a harmonic radar electronic license plate for motor vehicles incorporating a single antenna for receiving electromagnetic radiation and for emitting a unique pulse coded electromagnetic radiation at a harmonic frequency of the received electromagnetic radiation. U.S. Pat. No. 4,063,229 described a harmonic radar anti-shoplifting system incorporating a fusible link or other means in the electronic circuit of a tag to be incorporated into goods for sale so that the tag could be deactivated at the store's checkout counter before the goods passed the radar transmitter/receiver located at the exit(s) of the store.
Steel pipes, vessels and structural members (such as I-beams) are commonly used in industrial installations and are frequently covered with insulation. Inadvertent latent corrosion of such pipes, vessels or structural members can occur under the insulation which corrosion can be expensive to repair and can even end the useful life of the pipe, vessel or structural member. Therefore, it is common practice to periodically remove a portion of the insulation to inspect for such corrosion. Such inspections are expensive and invasive of the integrity of the insulation. It would be an advance in the art of such inspections if a non-invasive remote wireless inspection means were devised.